In Situ Electrical Transport and X-Ray Spectroscopy Study in Spintronics
This proposal is to use a combined electrical transport and x-ray spectroscopic technique (called in-situ E-S characterization henceforth) that we recently developed at NSRRC, to investigate two topics related to spintronics. The technique allows us to perform x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) with simultaneous transport characterizations, enabling research concerning electrical transport, element- and orbital-selective magnetization, with an in-situ fashion. It is a unique approach to real-time change of electronic state for a material/device exhibiting magneto-electric responses. Using this technique, we plan to investigate the electrical (E)-controlled magnetic properties of CoFeB-MgO based magnetic tunnel junction (MTJ), and spin-transport of Ti1-xCoxO2 dilute magnetic semiconductors. For CoFeB-MgO MTJ, we will focus on the effects of Co(Fe)-oxygen p-d hybridization on the perpendicular anisotropy and E-controlled magnetic properties, probing the nature of Co(Fe) spin-orbital coupling involving in the anisotropy reversal. Recent works speculated that the interfacial hybridization in CoFeB/MgO, as well as in other ferromagnetic/high-k oxide interfaces determines their E-controlled magnetic properties. However, the understanding of the phenomenon remains at the step of speculation, yet E-induced change in 3d (2p) electronic states leading to the magnetization change, hasn’t been experimentally observed. In this work, we plan to harness the in-situ E-S characterization to probe 3d (2p) electronic modifications of CoFeB (MgO) in response to applied E-field. This will point to whether the ferromagnetic d-band electron filling leads to magnetic manipulation in CoFeB via interfacial hybridization. Using CoFeB-MgO as an example, the work will provide a conclusive interpretation to the studies dealing with E-controlled magnetic properties over recent years. For Ti1-xCoxO2, we are aimed at understanding the fundamental difference between Ti1-xCoxO2 and defective TiO2, in terms of induced ferromagnetism persistent to room temperature. Although recent discoveries in defective TiO2 suggested that the ferromagnetism originates from the defects, a much lower Curie temperature and the absence of anomalous Hall effect (AHE) of the defective TiO2, in comparison to Ti1-xCoxO2, implying that their induced ferromagnetisms are fundamentally different. In this work we will take advantage of the technique to probe the AHE in the presence of Co 3d – O 2p exchange interactions. We will also investigate the Co-dependency of Co 3d – O 2p exchange interactions along with the AHE, to see the influence of Co concentration on the exchange and transport effects. For defective TiO2, we focus on understanding the Ti -O coupling originating from the Ti 3d – O 2p hybridization, which highly depends on the defect forming. The fundamental difference between Ti1-xCoxO2 and defective TiO2 could be understood by identifying the induced moment quantities, associated electronic states, and concentration/mobility of carriers. The success of the work will provide a deep understanding of interplay between charge, spin and orbital degrees of freedom in spintronics and bring considerable impact to the field.